The present invention relates to the field of synthetic gemstones. More specifically, it relates to a method of preparation of a synthetic fire opal that has similar physical and chemical properties as a natural fire opal.
Natural opal is a gemstone made of hydrated amorphous silica (chemical formula SiO2.nH2O). Based on visual appearance, natural opals can be classified as precious opals, common opals, and fire opals. Precious opals exhibit a ‘play-of-color’. Play-of-color is the phenomenon of changing spectral hues produced by the diffraction of white light through a microstructure of orderly arrayed-silica spheres. Common opals have high opacity and do not exhibit the play-of-color. Fire opals are transparent or translucent opals with an orange or red body color.
Natural fire opal is an opal that has been mined from the earth and may be treated in a number of ways to enhance its qualities. Natural fire opals are rare and expensive. Also, in natural opals, there is a lot of variation in color from piece to piece. Natural fire opals have long been used as ornamental stones in jewelry. However, natural fire opals are brittle and heat sensitive. Most natural fire opals also show crazing. Crazing is the phenomenon of developing cracks in the opal with the passage of time or when the fire opal is subjected to heating.
Simulants of natural opals exist in the market. These simulants are used as alternatives for natural opals. Simulants are artificial stones that resemble the natural opal, but their chemical composition and physical properties are not the same as natural opals. Synthetic opals have the same chemical composition and physical properties as natural opals. Scientists have been able to produce synthetic opals in laboratory conditions.
A paper titled ‘Some Aspects of Precious Opal Synthesis’, by S. V. Filin, A. I. Puzynin, and V. N. Samoilov, published in ‘Australian Gemmologist’, Vol. 21, No. 7, 2002, pp. 278-282, describes one such method of preparation of synthetic opals. The paper describes the synthesis of an opal by a multi-step process. In the first stage of the process, synthesis of monodisperse particles of silica of about 300 nm size is carried out by the hydrolysis of tetraethyl ester of orthosilicic acid, Si(OC2H5)4 in ethanol. In the second stage, the raw precursor of opal is precipitated by spontaneous sedimentation. The sedimentation time is about seven months. In the third stage, the precursor opal is dried by a method of supercritical drying in an autoclave to obtain samples of any dimension within a time span of 10-12 hours. This is followed by sintering in a furnace at temperatures of 600° C., 800° C., and 1000° C. The total time involved in the synthesis of a synthetic opal is 10 months.
U.S. Pat. No. 4,703,020, titled ‘Process for the Production of Jewelling and Ornamental Material’, assigned to Kyocera Corporation, Kyoto, Japan, describes another such method of production of synthetic opal. This patent describes a process of production of a jewelling and ornamental material which produces a play-of-color similar to a natural opal. The main raw material used is amorphous silica spheres with a particle diameter within the range of 0.15 to 0.4 microns. The mixture of amorphous silica spheres and water is subjected to sedimentation for several weeks to several months. The resulting jelly-like precipitate is spontaneously dried, and then calcined to a temperature of about 800° C. to form a structure composed of regularly three-dimensionally arranged amorphous silica spheres. Zirconium alkoxide, in the form of a solution, is impregnated in the three-dimensional structure. The entire structure is then calcined at a temperature of about 1150° C. for about 30 hours to obtain a jewelling and ornamental material.
The above-mentioned methods of preparation of synthetic opals depend upon the three-dimensional arrangement of amorphous silica spheres to produce play-of-color. Therefore, such methods may not be applied for the preparation of synthetic fire opals without play-of-color. In addition, such methods may not lead to the production of synthetic fire opals in different colors. Another limitation is the time taken by each of these processes. The entire process in the existing methods of preparation of synthetic opals takes about 7-10 months. Further, they also require production of amorphous silica spheres with particle diameter in the specified range, as the first step in the process of preparation of a synthetic opal.
It is therefore evident that there is a need for a process of preparation of synthetic fire opals. The process should produce synthetic fire opals with color uniformity. The process should be less time consuming and should lead to the preparation of synthetic fire opals in different colors. Further, the process should produce synthetic fire opals, not limited by the particle size of the raw material.